Rabies Antibody Screen

About the RFFIT Test: RFFIT stands for Rapid Fluorescent Foci Inhibition Test. It is a serum neutralization (inhibition) test, which means it measures the ability of rabies specific antibodies to neutralize rabies virus and prevent the virus from infecting cells. These antibodies are called rabies virus neutralizing antibodies (RVNA).

In RFFIT, the serum (the non-cellular portion of blood) is first diluted fivefold (1 part serum in 4 parts diluent). Further (serial fivefold) dilutions are performed, each of which contain less and less serum. These serum dilutions are mixed with a standard amount of live rabies virus and incubated. Whatever RVNA is present will neutralize the virus. Next, cultured cells are added and the serum/virus/cells are incubated together. Whatever rabies virus is left (i.e., that which has not been neutralized by the RVNA), will infect the cells and this can be seen under the microscope through the use of specific staining. Calculation of the endpoint titer is made from the percent of virus infected cells observed on the slide.

KSVDL provides both the World Health Organization (WHO) guideline of 0.5 International Units per milliliter (IU/mL) as an adequate response to rabies vaccination, as well as the ACIP level of “complete neutralization at a serum dilution of 1:5 in the RFFIT test” as evidence of adequate response in a person on our RFFIT antibody test reports. We provide both guidelines to better assist individuals and healthcare providers in their decision-making about how to interpret our tests results in light of vaccination status and the possible necessity of booster vaccination(s).

According to World Health Organization guidelines, a rabies antibody level of greater than or equal to 0.5 IU/mL demonstrates an adequate response to vaccination (3). If the level falls below this value, a booster dose of rabies vaccine may be recommended for people who are at frequent risk of rabies virus exposure. In contrast, the ACIP guidelines state that evidence of complete neutralization at a serum dilution of 1:5 in RFFIT testing (corresponding to 0.1-0.2 IU/mL in our laboratory) is considered an adequate response to rabies vaccination (4). The lowest antibody level that can be accurately and precisely measured by the RFFIT in the KSVDL laboratory is 0.1 IU/mL; below this level, there is uncertainty as to the specificity of the result. Because the ACIP level is close to the assay threshold, the level of 0.5 IU/mL is more conservative for guiding human vaccination decisions and applicable in most situations.

The Rabies Antibody Screen is a qualitative result. Results are reported as either <0.1 IU/mL, >/= 0.1 IU/mL, or >/= 0.5 IU/mL.